In the papermaking process known as Kraft pulping, the pulp yield and reject level are a function of the degree of delignification. The lignin in wood chips is chemically attacked and split into fragments by the hydroxyl (OH.sup.-) and hydrosulfide (SH.sup.-) ions present in the pulping liquor. The lignin fragments are then dissolved as phenolate or carboxylate ions. This chemical reaction is known as delignification.
It is believed that penetration and diffusion are two major functions involved in the delignification process. In many cases, insufficient penetration causes higher rejects and a lower degree of cooking because the cooking liquor moves much more rapidly in the longitudinal direction (by penetration) than in the transverse direction (by diffusion) of the fibers.
Therefore, the reject reduction and total yield can be improved by enhancement of penetration of cooking liquor into the wood chips. Three parameters are responsible for the function of penetration. They are: (1) interfacial tension, (2) surface tension, and (3) contact angle.
Interfacial tension may be defined as the work required to increase the unit area of an interface at constant temperature, pressure and composition. Surface tension is the interfacial tension between the liquid and the air or the solid and the air, and contact angle is defined as the angle formed by a droplet in contact with a solid surface, measured from within the droplet.
The interfacial tension between the cooking liquor and resin must be dramatically decreased in order to increase the penetration rate of cooking liquor into the wood chips. Two mechanisms are involved with the lowering of interfacial tension: deformation of resin and formation of an emulsion or microemulsion.
Low interfacial tension reduces the work of deformation necessary for resin droplets to emerge from the narrow necks of pores. A very low liquor/resin interfacial tension allows resin to move easily through the necks of pores. This mechanism can assist in the penetration of liquor into the chips.
Alternatively, a very low interfacial tension is required to form an emulsion or microemulsion of resin in the cooking liquor. If resin, which blocks the pores, can be emulsified by a surfactant, the cooking liquor can pass easily through the pores. This leads to improved liquor penetration.
The increased wettability of a chip surface by a surfactant also creates more favorable conditions for cooking liquor penetration. The spreading of cooking liquor on the chip surface is governed by the surface tension of the cooking liquor, the the surface tension of the chip, and the interfacial tension between the cooking liquor and the chip. The tendency of spreading cooking liquor on the chip surface is indicated by measuring the contact angle of the liquid on the chip surface. In general, the lower the contact angle of the cooking liquor, the easier spreading occurs. Ease of spreading can be accomplished by adding the proper surfactant to the cooking liquor.
Prior art references teach the use of ethoxylated alkylphenols (U.S. Pat. No. 4,952,277) and ethylene oxide-propylene oxide block copolymers (U.S. Pat. No. 4,906,331) as Kraft pulping additives.